1. Field of the Invention
In recent years, optical application technologies such as optical communication, optical information processing, and optical measurement have been rapidly developed, and a great need exists for the development of wavelength conversion technologies for an optical signal.
The present invention relates to a method and apparatus for wavelength conversion of signal light for converting a wavelength .lambda.s of signal light to an arbitrary wavelength of .lambda.1 to .lambda.n, and an optical transmission scheme using the apparatus.
2. Description of the Related Art
Technologies as mentioned above for converting the wavelength of an optical signal include, for example, a semiconductor optical device disclosed in Laid-open Japanese Patent Application No. 6-302903 (JP-302903/1994). The configuration of this semiconductor optical device is shown in FIG. 1. Referring to FIG. 1, the semiconductor optical device is provided with slab optical waveguides 13A, 13B of fan shape at both ends of array waveguide 16 which comprises a plurality of waveguides having delay time difference. One slab optical waveguide 13A has focal surface 14A connected to a plurality of gain waveguides 17. The other slab optical waveguide 13B has focal surface 14B connected to saturable absorption waveguide 20 having a saturable absorption area through passive waveguide 19A and input/output gain waveguide 18B.
The device operates in the following manner. First, gain waveguide 17 supporting a required output optical signal wavelength is injected with a current a little less than a threshold value current at which laser oscillation is started. At this point, since spontaneous emission light emitted from gain waveguide 17 is absorbed in the saturable absorption area, the laser oscillation is suppressed. In this state, when an input optical signal is incident on input/output waveguide 18B, the light absorption is saturated in the saturable absorption area to generate the laser oscillation at a wavelength corresponding to the selected and conducted gain waveguide. This laser oscillation repeats on and off following the input optical signal, thereby causing the semiconductor optical device to serve as a wavelength conversion element. The outputted wavelength is instantaneously switched by switching the conducting gain waveguide 17.
Also, in JP-102643/1997, an optical wavelength conversion circuit of oscillation suppression type as shown in FIG. 2A and an optical wavelength conversion circuit of saturable absorption type as shown in FIG. 2B are disclosed.
In the optical wavelength conversion circuit of oscillation suppression type in FIG. 2A, semiconductor laser 11 in free oscillation at wavelength .lambda.2 by injecting a current at an oscillation threshold value or more is applied with intensity modulated signal light at wavelength .lambda.1. The oscillation of the semiconductor laser is suppressed when the applied signal light is on, thereby outputting wavelength converted signal light at wavelength .lambda.2 which forms a complementary signal train to the applied signal light at wavelength .lambda.1.
In FIG. 2A, the optical wavelength conversion circuit comprises DBR (Distributed Bragg Reflection) type semiconductor laser 11 in which optical amplifying portion 11c having no polarization dependency as a gain area and oscillation polarization selecting portion 11d are disposed in a resonator which has DBR area 11a serving as an oscillation wavelength selecting portion and end surface 11b as two reflecting surfaces. In the respective areas, currents are injected from corresponding electrodes 10a, 10b, and 10c respectively.
In the optical wavelength conversion circuit of saturable absorption type in FIG. 2B, semiconductor laser 12, having a saturable absorption portion injected with a current at an oscillation threshold value or less and an optical amplifying portion injected with a current to the extent that it oscillates at a wavelength .lambda.2 when the absorption of the saturable absorption unit is reduced, is applied with intensity modulated signal light at a wavelength .lambda.1. The absorption of the saturable absorption portion is reduced to cause oscillation at wavelength .lambda.2 when the applied signal light is on, thereby outputting wavelength converted signal light at wavelength .lambda.2 which makes a signal train with the same sign as the applied signal light at wavelength .lambda.1.
Semiconductor laser 12 is a DBR type semiconductor laser in which saturable absorption portion 12c having no polarization dependency and optical amplifying portion 12d of polarization dependent type are disposed as a gain area in a resonator which has DBR area 12a and end surface 12b as two reflecting surfaces. Saturable absorption portion 12c and optical amplifying portion 12d have a property that the polarization dependency is low when a low current is injected and the polarization dependency is high when a high current is injected. Saturable absorption portion 12c serves as a saturable absorption area with no polarization dependency in a low current injected state. Optical amplifying portion 12d functions as a gain area, in which laser oscillation is started in a single mode by the mode gain difference dependent on the polarization when the absorption of saturable absorption portion 12c is reduced by the applied signal light being on in a high current injected state.
The operation of this circuit is as follows. The intensity modulated signal light at the wavelength .lambda.1 is applied to saturable absorption portion 12c with no polarization dependency. In saturable absorption unit 12c, carrier density is increased by stimulated absorption of the applied signal light, regardless of the polarization. With this, the loss in the laser resonator at the wavelength .lambda.2 is reduced, resulting in the oscillation at the wavelength .lambda.2 only when the applied signal light is on. In other words, the outputted level of the oscillation light at the wavelength .lambda.2 is modulated in the same sign in accordance with the on and off of the applied signal light at the wavelength .lambda.1, and the wavelength conversion is performed from the wavelength .lambda.1 to the wavelength .lambda.1.
An optical device having a saturable absorption area, hereinafter referred to as an SA (Saturable Absorber), has a saturable absorption effect caused by applying a semiconductor waveguide with a reverse bias, and is also referred to as an optical gate. Specifically, the SA has a nonlinear transmission characteristic dependent on the intensity of applied light, in which incident signal light with a low intensity is greatly absorbed and reduced, whereas incident signal light with a higher intensity causes a reduction in the absorption coefficient and exceeds the absorption capacity of the optical device to be transmitted. For realization of such SA, for example, Hashimoto et al. reported the realization thereof by applying a semiconductor laser amplifier with a reverse bias. (Y. Hashimoto et al. Technical Digest of CPT98, pp 215-216, Jan. 12-14, 1998).